1. Field of the Invention
The present invention relates to the announcement of media stream connections for a media session over a communications network.
2. Related Art
Multicast transmissions are becoming increasingly common on the Internet. In contrast to standard Internet Protocol (IP) point to point transmissions (unicast), IP multicast allows the simultaneous transmission of information to a group of recipients from a single source. Routing support for IP multicast transmissions is provided by the MBone (IP Multicast Backbone) which is a virtual network layered on top of the Internet.
IP multicast allows real-time communications over wide area IP networks and typical transmissions include video and audio conferencing, live multimedia training, university lectures and transmission of live television programmes.
A multicast transmission usually consists of a multimedia session made up of several individual media streams typically carrying video, audio, whiteboard or raw data. Some sessions are persistent, but the majority exist for a specific period of time, although need not be continuous. Multicast based transmissions on the MBone differ from unicast IP transmissions in that any user receiving the transmission can join the session (unless the transmission is encrypted) and to receive a transmission, a user need only know the appropriate transmission address and timing information.
Prior to a multicast transmission an appropriate announcement containing a session description is made, usually at an IP group multi-cast address. Standard session descriptions are generated using a Session Description Protocol (SDP), as defined in the Internet Engineering Task Force's draft RFC 2327. SDP is a simple ASCII text based protocol that is used to describe real time multimedia sessions and their related scheduling information. SDP messages are wrapped in a carrier protocol, known as a Session Announcement Protocol (SAP), which, in addition to containing the necessary IP addressing and routing information for transmission across the Internet or MBone, allows the SDP message to be encrypted, signed or compressed. An announcement can then be sent at regular intervals to the announcement group address. As an alternative to SAP, a session may be announced by placing an SDP message on a World Wide Web site (WWW) or by sending it to individuals by email or as a unicast transmission inviting them to participate.
An SDP message conveys information about each media stream in the multicast multimedia session to allow the recipients to participate in the session. A typical SDP message will include the session name and purpose, the time(s) and date(s) the session will be active, the component media streams of the session and information required to participate in each media stream (IP multicast address, port, media format). The SDP message may also include details of the session's bandwidth requirements, an encryption key necessary to participate in a secure multicast transmission using public key encryption, contact information for the organiser of the multicast session, and a Unique Resource Indicator (URI) pointing to a WWW or an Intranet web site where further information on the session may be found, for example, background information relating to the conference.
The level of participation a user may make in a session or stream depends on its purpose. In a multicast television session, typically users would only be able to receive the session streams whilst in a multicast conference session the communication would be bi-directional with a central server (such as group address 120) receiving each participants transmissions and relaying them to the other participants. The level of participation expected of a user in a session or stream may be explicitly stated in the session description or it may be inherent from the session description, for example when a receive-only application is associated with a media stream type in the session description.
A common front end interface used by multicast end users is known as Session Directory Rendezvous (SDR). This interface takes the received announcements, decodes the SDP message and displays the names of those sessions that are still current in a list. The end user may then select one of the listed announcements to view further technical and user-oriented details of the announced session. From the displayed information, the end user can then select to join individual streams of the session or to join the entire session. Once the streams to be joined are selected, SDR starts the necessary multicast-enabled multimedia application on the end user's computer, such as Vic and Vat, and passes the relevant stream information (a transport port address) from the announcement onto the application allowing the application to establish the link to the associated IP multicast address and participate in the stream at transmission time. Having initiated the applications and passed the relevant transport port address SDR plays no further part in the session.
Recent increased usage and demand for (multi)media sessions has highlighted a number of limitations in SDP. SDP limits session descriptions to defining a session having a single set of timings that apply to all of the streams within it. A session in which a stream starts mid-way through the transmission cannot easily be described using SDP. The structure of a session description written in SDP must be a simple linear list of streams which may not reflect the intuitive structure of a complex session. SDP supports a limited and predefined set of applications which can receive the streams and a limited and predefined set of transport mechanisms (e.g. Simple layering, RTP and UDP). As guaranteed Quality of Service (QoS) is becoming more and more desirable to the consumer and the supplier, the need to define QoS policies for the entire session and individual streams in terms of required system resources, bandwidth requirements and supported applications also needs to be met. There may also be requirements on the prioritisation of streams and subsessions or more complicated rules about receiving streams. A further requirement on the part of the supplier will be the need for charging facilities permitting the charging of an end user for a multicast transmission to which they subscribe according to the QoS and types of streams received etc. There is little scope to include information about QoS policies or charging within the conventional structure of an SDP session description, or any metadata about the session.